Calibration of the absolute spectral sensitivity of a solar UV radiometer with the use of synchrotron radiation

Nuclear Instruments and Methods m Physics Research North-Holland

A308 (1991) 165-168

165

Calibration of the absolute spectral sensitivity of a solar UV radiometer with the use of synchrotron radiation S.I. Anevsky a, A.E. Vernyi a, D .A. Gonyukh b, T .V. Kazachevskaya °, IN. Koriev V .I . Sapritsky a, V.B . Khromchenko a and Yu.N . Tsigel'nitskii

a

° All-Union Research Institute for Optical and Physical Measurements, 103045 Moscow, USSR n TsKB GMP, 249020 Obmnsk, USSR ` Fedoroa Applied Geophysics Institute, 129128 Moscow, USSR

The absolute spectral sensitivity calibration of a solar UV radiometer, intended for use on board a satellite for measuring solar illuminance at the Hry . wavelength of 121.6 rim, has been carried out against a standard based on a synchrotron radiation source . The estimated overall calibration error is 2.8% (rms). One of the major problems which gave impetus to the development of vacuum UV radiometry is the calibration of the absolute spectral sensitivity of radiometers, spectrophotometers and other radiation transducers mounted on satellites for the study and forecast of the state of the ozone layer, and the Earth's ionosphere and other investigational purposes. The main method for spectral measurements in vacuum ultraviolet (VUV) is the use of synchrotron radiation (SR) [1,2]. Regardless of the diversity of the available techniques employing storage rings and synchrotrons, all of them possess certain drawbacks associated both with the use of synchrotron radiation and with the calibration of the radiation receiver against a reference source . VUV radiometry is characterized by the possible loss of accuracy and even credibility of measurement results at each step of the calibration and use of the means of measurement. It is therefore necessary to design a simple and reliable technique to be used in this hard-tomeasure spectral region . This paper is concerned with the analysis of the results of the implementation of a rather accurate calibration technique for the absolute spectral sensitivity of a solar UV radiometer at the H,JY , wavelength of 121 .6 rim intended for use on board a satellite. Over 85% of the ionization optical solar radiation is concentrated on the H,y, line . Modelling of the outer region of the Earth's atmosphere and predicting its variations require the irradiance to be measured with an accuracy of several percent [3]. To isolate the H ry . line, one may employ a radiation receiver based on thermophosphorus CaSO4 (Mn) with a magnesium fluoride window . Such a combination provides a working spectral range of 115 to 150 rim with a peak sensitivity near 0168-9002/91/$03 .50 O 1991 -

123 rim. The design of the on board UV radiometer SUFR-Sp is described in ref. [4]. The calibration of the radiometer is aimed at finding its absolute spectral sensitivity over the 115-150 rim wavelength range in the working regimes, at studying the stability and reproducibility of its characteristics, and at measuring its sensitivity threshold on the H,,y, line . The calibration of the absolute spectral sensitivity of the radiation receiver over the 115-150 rim wavelength range is hindered, primarily, due to the lack of highly accurate reference receivers, since single-atom gas ionization chambers are not used at wavelengths above 102.2 rim. Application of a standard SR source with a continuous spectrum for the calibration of the absolute spectral sensitivity of the receiver requires the employment of a monochromator with a known spectral transmission factor. The problem may actually be solved with the use of an additional monochromator which should be capable, maintaining its positioning m vacuum, of turning around the incident beam to take account of the variation between the radiation fluxes polarized in the plane striking the lattice and in the plane perpendicular to it . This method required a great deal of effort to attain an acceptable accuracy and to avoid gross errors . The main sources of such errors are the zone nonuniformity of the diffraction lattice efficiency, degradation of the efficiency under hard SR and other effects, and violation of their positioning when turning the monochromator in vacuum. The calibration of the radiometer SUFR-Sp was accomplished in compliance with a technique chosen for the purpose, which is characterized by a convenient separation into relatively simple operations readily lend-

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1 . SR SOURCES/FEL

166

S I Aneosky et al / Calibration of a solar UV radiometer

ing themselves to metrological analysis and realizable with a high accuracy . At the first step, when the UV radiation produced by the synchrotron TROLL-1 [5] was used as a spectral radiance standard, the intensity of the radiation of five resonance lines produced by krypton, xenon and hydrogen discharge lamps was measured over the 115-150 rim working range of the spectrum . At the second step, the absolute spectral sensitivity of the radiometer SUFR-Sp was measured by the integral resonance lamps with the use of a series of filters pre-certified to their spectral transmission factors over the operating wavelength range. The optical scheme of the first calibration step is similar to that described in ref. [1]. The first calibration step is described by the following expression : I(X)

-L(Xo)i(R)iSR (Xo) k(Xo)

i

(X)i(Xo)

k(X)

,r(X) OSg(Ä, XO E, R, S) f xf(Ä) dÄ A where I(A) L(,N o ) 10), 100)

i sR (X) i sR (Xo) k(N), k(Xo)

is the radiation intensity of the resonance lamp on line X over the VUV range; is the spectral radiance of the tungsten strip lamp in the visible range at wavelength Xo; are the comparator signals in measuring the radiation produced by the resonance and tungsten lamps, respectively ; are the comparator signals in measuring the SR ; are the reflectivities of the flat mirrors;

are the relative transmission factors of the monochromators pertaining to the radiation polarized in the orbit plane and in the perpendicular plane at wavelengths X and a o , respectively ; XO E, R, S) is the relative spectral density of il(X, the SR flux at wavelengths X and X o determined by the energy of particles E, orbit radius R and the final axial, radial and phase dimensions of electron bunch S; is the hardware function of the P X) VUV-monochromator; is the spectral interval isolated by the VUV-monochromator; and AS is the area of the radiating region of the resonance lamp . In accordance with eq . (1), calculation of the relative spectral density of the SR flux requires the determination of the particle energy, orbit radius in the point of radiation as well as axial, radial and phase dimensions of the electron bunch. The number of accelerated particles was eliminated from the calculation by introducing a spectral procedure aimed at comparing the spectral radiance of synchrotron radiation and of the radiation produced by the tungsten band lamp to the visible range [5]. Under the conditions of the axial-symmetric magnetic field of the TROLL-1 accelerator, the radius of the equilibrium orbit is determined by the frequency of the accelerating magnetic field. The energy of the particles is found by an optical method employing a nonlinear correlation between the relative spectral density of the SR flux and the electron energy [6]. It is common knowledge that the major contribution to the distortion of SR characteristics of a real-life source, as compared to the characteristics of a pomtwise radiator, is introduced by the axial betatron oscillations

Table 1 Basic components of errors in finding resonance lines radiation power with the use of synchrotron radiation in the 115-150 rim wavelength range Error sources

(1) Calculation of relative SR spectrum with allowance made for errors in particle energy, orbit radius as well as axial, radial and phase dimensions of the bunch (2) Measurements of relative polarization factors in the VUV region (3) Measurement of relativity of the mirror m the VUV region (4) Measurement of the comparator pulsed signals (5) Calibration of the tungsten lamp against the black body model (6) Definition of the VUV monochromator hardware function (7) Other components Total error in finding the resonance lines radiation power in the 115 to 150 rim wavelength range

rins error estimate [%] 1.4 1 .2 0.8 0.4 0.3 0.3

0 .5

S./ Aneosky et al / Calibration

of a solar

167

UV radiometer

Table 2 Calibration of the absolute spectral sensitivity of the radiometer SUFR-Sp Designation of lamps, inflation

Anode current

Filament current [A]

Resonance line [nm]

Radiation power [mW/sr]

KsR-1 P (xenon)

200

3

KrR-1P (krypton)

200

3

LGV-1 (hydrogen)

5

-

129.6 147 116.6 123.6 121 .6

014 220 0 .40 1 .60 4 .88 X 10 -2

of electrons. The best mode from the metrological viewpoint is the "large bunch" mode, when the angular distribution of electron velocities within a bunch appears to be greater than the angular distribution of synchrotron radiation of a single electron [6]. The ratio of polarization factors 7 in eq . (1) is determined with the help of a mirror polarimeter [7]. Estimates of all mayor components of the error is finding the radiation intensity of resonance lamps are summarized, in rms form, in table 1 . The tabulated results serve as an overall characteristic of the synchrotron TROLL-1 as a reference SR source. At the second step, measurements of the spectral sensitivity of the SUFR-Sp radiometer were taken with the help of resonance lamps. The resonance lamp radiation flux was isolated by the aperture diaphragm so that the operating solid angle maintained its magnitude selected in the calibration with the use of SR. To now find the receiver spectral sensitivity, it suffices to separate the contribution introduced into the signal by the resonance lines radiation flux . Since xenon and krypton lamps each radiate two lines which fall into the operating spectral range, one should take extra measurements using a filter, for example, of magnesium fluoride, so that a system of two equations of the following form is obtained : ,(X)F1F =1

(X )Si ( X ) 02,

(2)

where i is the signal of the calibrated radiation receiver ; j and n are the line index and the total number of lines in the lamp spectrum, respectively ; F, (A) is the radiation intensity of the 7th line ; 'ri(X) is the transmission factor of the filter at the 7th line ; SI(A) is the receiver spectral sensitivity at the 7th line we have to find ; and AQ is the solid angle. An analysis of the stability of the solution to the system of equations (2) shows that with n = 2 the transmission factor of the filter should not vary within the

Filter transmission factor

SUFR-Sp sensitivity IV/W]

0.62 0.79 003 0.49 036

0 .51x10' 4 .5 X10 2 0 .81 x 10 7 1 .15 X 10 7 1 .12 x 10 7

operating wavelength range by more than 20%. To prove the fact that the contribution of the xenon or krypton continuum and the molecular spectrum of hydrogen to the signal is negligibly small, some extra synchrotron measurements of the spectral radiant power of the lamps have been taken in the entire operating wavelength range from 115 to 150 rim. Table 2 presents the type of lamps that were used, their operating currents, the lines radiant power measurement results, the transmission factors of the magnesium fluoride filter and the obtained values of absolute spectral sensitivity for the radiometer SUFR-Sp. The overall calibration error, in compliance with eq . (2), is a sum of four components amounting, with regard for partial derivatives, to 2.8% (rms). The stability and reproducibility of the sensitivity characteristics of the radiometer SUFR-Sp have been studied, using the synchrotron TROLL-1, systematically during three years. The annual deviation of the absolute spectral sensitivity did not exceed the calibration error. The sensitivity threshold of the instrument was found to be approximately 10-12 W with an accumulation time of 12 .5 s. The SUFR-Sp solar radiometer has been installed on the Prognoz-10-Intercosmos satellite and measurements in the spectral region X < 130 rim and at the HLy. line were carried out during a period of minimum solar activity, thus the short-wave radiation flux was low enough . The intensity of radiation at the HL .. line varied from 3.4 X 10 -4 to 4.6 X 10 -3 W/cm2 , depending on the presence of active areas on the sun. According to our results, the mean intensity was 3 .7 X 10 --3 W/m2 . This value is in good agreement with the value 3.9 X 10 -3 W/m2 obtained from measurements carried out on the US satellite Solar Mesospheric Explorer [8] . In 1988, similar instruments, SUFR-Sp-F, calibrated in the above described manner have been installed aboard the FOBOS-1 and FOBOS-2 satellites intended to measure solar radiation during the flight to Mars . In conclusion, the optimization of the bunch parameters obtained from the new reference SR source, 1.

SR SOURCES/FEL

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168

TROLL-2 [9], opens the door to an increased radiometer calibration accuracy in order to attain a metrologically better SR generation regime .

References [1] M. Kühne, F. Riehle, E. Tegeler and B. Wende, Nucl Instr. and Meth . 208 (1983) 399. [2] J.Z Kloze, J.M Bridges and W R Ott, J. Res. Nat. Bur. Stand. 93 (1988) 21 . [3] DT Heath, m : Solar Energy Flux and Its Measurements, ed . O. White (Colorado Ass Univ . Press, Boulder, CO, 1977)

[41 TV Kazachevskaya, D.A . Gonyukh and G.S IvanovKholodnyi, Geomagnehzm i Aeronomrya 25 (1985) 996, m Russian S I. Anevsky, A.E . Vernyi, V 1. Kvochka, IN . Konev, A V. Makul'km and V.S . Panasyuk, Izmentel'naya Tekhmka No 12 (1987) 4, m Russian. [61 S I Anevsky, A.E. Vernyi, V.S Panasyuk and VJ Sapntsky, Phys Scnpta 35 (1987) 623 R.N . Hamm, R.A . Mac Rae and E.T . Arakawa, J. Opt . Soc Am . 55 (1965) 1460 . [81 G.I . Rottman, COSPAR Report (Helsinki, 1988) p 12 .2 .1 [9] S 1 Anevsky, A.E. Vernyi, V.S . Panasyuk and V.B Khromchenko, Nucl Instr. and Meth A261 (1987) 56